1. Field of the Invention
The present invention relates to isometry testing devices, and more particularly to devices for determining the isometric point of a knee cruciate ligament replacement in a femur after a tibial tunnel has been prepared by attaching a fastening means to an estimated placement point on the femur with strand means, such as a heavy suture, attached to the fastening means and extending away therefrom to be utilized with the device.
The manner in which cruciate ligaments are replaced is well known and well documented. In our copending patent application Ser. No. 07/650,680, filed Feb. 5, 1991, titled Variable Angle, Selective Length Tibial Drill Guide, we discussed the location and preparation of the tibial tunnel in which the tibial portion of the cruciate ligament replacement is anchored. After such a tibial tunnel is prepared, a similar tunnel for anchoring the femur portion of the cruciate replacement in the femur is prepared. The tunnel in the femur must be located carefully with respect to the tibial tunnel and surgeons have found that the femur tunnel should begin at a point which is referred to as the isometric point on the femur. This isometric point is well known and well described in the literature. Reference is made to an article by Ben Graf, M.D., University of Wisconsin, Madison, Wis., titled Isometric Placement of Substitutes for the Anterior Cruciate Ligament. Dr. Graf's article outlines the value of locating the isometric point and using that point to locate the femur tunnel for anchoring the femur portion of the cruciate ligament replacement. Dr. Graf's article shows how the point may be located and proposes a device for testing the isometry of any selected point, the device being shown in sectional view if FIG. 1 and in use in FIG. 2.
2. Prior Art
This article by Dr. Graf and the device and method disclosed therein are referenced in detail not only to discuss isometric placement, but to discuss the presently known prior art devices over which the present invention is a significant improvement. For instance, the device shown in Dr. Graf's article clearly is a tension application device utilizing a spring to apply the desired tension to the strand means (suture) attached to a estimated isometric point with an excursion measuring means to show the movement of the strand means when the knee is flexed. As explained by Dr. Graf, when this movement of the strand means is less than or equal to 1.5 mm, the isometric point is very close to or on the point to which the strand means is fastened. It is necessary to place the strand means under a given tension because the cruciate ligament and the replacement cruciate ligament will be under tension in the knee. This tension causes the knee to react during flexure as it would with a ligament in place.
The prior art, therefore, teaches attaching a suture or other flexible strand means to what Dr. Graf refers to as the "initial point selection" and then connecting that suture to a spring-loaded tension gauge which will read the excursion movement of the strand means when the knee is flexed. Another example of such a device is a device offered by MEDmetric Corporation which comprises a tension gauge that measures excursion. The MEDmetric device comes equipped with a plurality of tool noses which fit on the front of the device to engage it with the knee. Acufex Microsurgial, Inc. has a brochure out showing what appears to be the device disclosed in Dr. Graf's article discussed above.
Further reference is made to an article in the Journal of Orthopedic Research by D. I. Bylski-Austrow, et al. titled Anterior Cruciate Ligament Replacements: A Mechanical Study of Femoral Attachment Location, Flexion Angle at Tensioning, and Initial Tension. This reference clearly teaches the importance of precise placement of cruciate ligament replacement anchoring points.
Finally, U.S. Pat. No. 4,632,100 discloses a fastening means to which a suture is attached, but it will be appreciated that the present invention does not involve any particular type of fastening means or strand means.
In summary, therefore, it is well known that the isometric point must be carefully located and that it may be located by attaching flexible strand means to an estimated point and applying tension to that strand means while the knee is flexed to observe the excursion movement of the strand means.
The present invention is an improvement over what is known for several substantial reasons including a self-contained driver for the fastening means disposed in the device for ease of use with the device and for retraction into the device after the fastening means is attached. This feature permits only one entry to be made through the tibial tunnel to attach the fastening means and then to measure the excursion. Without the self contained, retractable driver, the surgeon would have to enter through the tunnel to fasten the devise and then to reenter the tunnel with the tension/excursion test device.
Another important feature of the present invention not shown in the prior art is the provision of a plurality of separate gauges attachable to the holder or handle portion of the device, each gauge being provided with a preselected tension capability to match the selected ligament replacement material.
Another important feature of the present invention not shown nor suggested by the prior art is that it is designed to have a first barrel portion which acts as a handle for the driver and a detachable second barrel portion which serves as the tension and excursion gauge. The importance of this feature will be more fully discussed hereinafter, but it will be understood by surgeons working in the field that such a device should be relatively small in diameter and compact to be easily held in a very confined surgical situation. It will be seen that the illustrated and preferred embodiment of the present invention is generally an elongated, cylindrically shaped instrument or tool ideally suited for use in what is referred to as arthroscopic surgery in which the surgeon works in a confined situation.